This invention relates in general to electrical circuits, and more specifically to a tunable filter.
One or more filters, usually in the form of pass-band filters provide the front-end selectivity for a radio receiver. Such filters either are designed to have fixed bandwidths or they may have tunable bandwidths. Tunable bandwidth filters are usually used in wideband receivers where the operational frequency range is large. Most tunable filters use varactors as the tuning element. Each resonator element in the filter includes one or more varactors that in response to a control voltage provides the tuning mechanism for the filter. By varying the control voltage, the filter may be tuned to provide a narrow bandwidth for a desired frequency of operation.
In FIG. 1 there is shown a prior art receiver 100. An antenna 102 is used to receive radio frequency (RF) signals which are sent to a first filter also known as a pre-selector 104. The main function of filter 104 is to limit the bandwidth of the spectrum reaching the radio frequency amplifier 106 in order to minimize IM distortion and to attenuate the receiver spurious responses (e.g., image, xc2xd intermediate frequency (IF), etc.). The typical selectivity for filter 104 is in the range of 35-40 dB. As such, if a high selectivity receiver is required, a second filter stage 108 is used to provide for another 35-40 dB of selectivity.
As is done in conventional receivers, the filtered signal is then multiplied by mixer 110 with a local oscillator (LO) signal 112. The result of this multiplication process is a pair of output intermediate frequencies (IFs) 114 that are the sum and difference of the signal and LO frequencies. One of these IFs 114 is selected as the desired IF which is then band-pass filtered and amplified (circuitry not shown). The amplified signal is then sent through a second mixer or detector which translates the IF signal to baseband (audio) where it is further amplified and filtered prior to being presented to the user.
The main problem with the receiver 100 is that in order to achieve high selectivity, for example, over 90 dB at the image frequency, two or more filter stages 104 and 108 are needed. The use of multiple filter sections 104 and 108 adds to the overall cost of the receiver, since more components, shields, etc. are required. The cost of adding such extra filter sections is even higher in tunable filters given that tunable filters need to use expensive components such as varactors. Therefore, there exists a need for a tunable filter that can provide high selectivity while minimizing the cost of the filter.